The use of optical phantoms has the advantage that it avoids many manipulations on laboratory animals such as mice or rats. Thus, the animals themselves are spared. There is also the saving of the time spent preparing for manipulations of this kind, which require considerable administration, permissions for obtaining and using systems appropriate to the animals for anaesthetizing them and keeping them unconscious during the operations.
These phantoms are used, in particular, for simulating the optical properties of tissues in ranges of wavelengths in the red and near infrared. In medical imaging, this wavelength range is interesting since the absorption of living tissues is less in this band of the spectrum. More precisely, a spectral window of interest is defined, located between the wavelengths of 600 nanometres and 900 nanometres.
Conventionally, the procedure for working in this band of the spectrum, as illustrated in FIGS. 1 and 2, is as follows. A specific marker M is injected in a patient or an animal S in the case of experiments (a mouse in FIG. 1). This marker M attaches specifically to the cancerous tumour T. The zone of interest is illuminated with a laser having an excitation wavelength in the red and a fluorescence signal is recovered. The laser excitation signal L is represented by a descending arrow in FIG. 1 and the fluorescence signal is represented by ascending arrows. This signal is a mixture of specific fluorescence FS (grey ascending arrows in FIG. 1) derived from the injected markers but also from autofluorescence FA (white ascending arrows in FIG. 1), i.e. natural fluorescence from biological tissues. In this wavelength range, the autofluorescence from tissues, per unit volume, is relatively slight, but it is sufficiently troublesome to drown the fluorescence signal, notably when the latter is weak. This is notably the case when a fluorophore is located deep inside a tissue.
FIGS. 2A, 2B and 2C show, as a function of the wavelength, the intensity I of the signal from specific fluorescence FS, the signal from autofluorescence FA and the measured fluorescence signal FT, which is composed of the two preceding signals. As can be seen in FIG. 2C, the measured fluorescence signal FT can be seriously disturbed by the signal from autofluorescence FA.
It is therefore useful, if a “phantom” is required to be representative of a biological tissue in a range of wavelengths, and notably in the window of interest defined above, if it also possesses fluorescence characteristics representative of the autofluorescence of the tissue. Now, the origin of the autofluorescence from tissues is not yet known completely and quite particularly in this range of red or infrared wavelengths.
The biological tissues have a complex structure and comprise numerous components. We may mention, among others, collagen and elastin proteins, various amino acids such as tryptophan, agents such as NADH (nicotinamide adenine dinucleotide), lipo-pigments, components such as pyridoxine, enzymes such as flavine and other components such as the porphyrins. All these components, whose molecular structure can be very complex, may fluoresce in the visible spectrum.
Moreover, spectral differences may be observed in one and the same living being. In fact, there are structural differences between the anatomic localizations which may be reflected in variations of the form and intensity of the autofluorescence spectra. These differences vary from one individual to another.
Consequently, it is not a simple matter to produce a “phantom” that is representative of an organ or of biological tissues possessing characteristics of autofluorescence identical or similar to those of organic tissues.